Thursday, May 22, 2008

Rim Heating During Hard Braking

While I don't know "how-much"brake shoe fins may help for their intended design (whatever that is), a general increase in surface area should aid heat dissipation/transfer to the air. I love it when readers help me fill my own limited knowledge. Take a look at the comments beside yesterday's post on brake shoe fins. See if you can come up with some more "possible" uses. :)

However, the subject of heat production in rims is real. I realize that a few riders out there may still not have come to terms with this. Allow me to wander and bore you.

Think of a 15% gradient downhill slope, narrow and curvy. Now think about your total weight,speed,drag co-efficient yada yada. Think about 60+ mph. Now think a sorry looking deer popping out of no-where just 200m in front of you. Rapid deceleration needed or you and the deer won't make it. Either deer will end up inside you or you'll end up inside the deer through the wrong side and it'll make for an interesting news article.

Picture a sorry looking deer

Hard relentless braking during such an emergency is a likely candidate for rim temperatures shooting beyond 70 degree C (150 F). Try something like this, stop the bike and then feel the rim yourself. I caution safety first and foremost. Carry an oven glove with you too. It may fit in the jersey pocket. I think Specializedmakes one.

In a chapter on 'Braking', in the book Bicycling Science (Gordon,MIT), there is a nice graph of rim temperature above ambient vs speed for a few cases consisting of different bicycles, drag co-efficients, combined masses and wheel diameters.

If I interpret it right, here's the interesting trend as shown by it though. As speed increases, rim heating decreases. More air flows past, hence more cooling and heat dissipation. Nothing hard there to imagine.

Such is the dilemma of downhill racers, says Gordon. Going fast on one hand avoids heating the rim whereas emergency braking on the other presents the danger of suddenly overheating it, making for a very dangerous situation.

How dangerous? If temperatures cross a point, they can soften the tire, blow the sidewall and even cause a tubular glue failure, allowing the tire to slip out. All this can be magnified by the high speed, improper front-rear wheel braking reactions, and poor worn out rims (think twice about buying used wheels from Ebay) and you'll be very lucky to survive if you crash. Some are in disbelief (mostly non-cyclists) when you tell them that cycling is one of the most dangerous sports there is.

In general, rim heating depends on the following, not necessarily in order of importance. I wont explain any of the following since I take it that you can apply a little of your own thinking to each.If I missed something, do point out.

1. Mass of rider+bike - More mass to stop, more dissipative energy.

2. Tire grip and rolling resistance.

3. Deceleration rate - % of g (acceleration due to gravity).

4. Braking surface area and wheel diameter.

5. Slope of road - Dictates speed and type of energy to be dissipated in the brakes. Physics 101.

Tim : I meant just the contrary. Deep-section rims should provide a streamlined flow of air while non-aero wheels can lead to separation in the wake, disrupting dissipation. And yes, they have greater surface area too. I don't know how this works out for carbon rims, I'm speaking for alloys. The dilemma I was referring to is the fact that these wheels can pick up speed so fast.